DENSITY-FUNCTIONAL STUDY ON THE PREACTIVATION SCENARIO OF THE DOTZ-REACTION - CARBON-MONOXIDE DISSOCIATION VERSUS ALKYNE ADDITION AS THE FIRST REACTION STEP

Two different mechanistic proposals have been investigated theoretical ly for the initial steps of the chromium-assisted Dotz benzannulation reaction at the nonlocal density functional level of theory. The energ y requirements needed for the reaction to start through the usually as sumed CO-dissociative mechanism are calculated to be 144.7 kJ mol(-1) for (CO)(5)Cr=C(OH)(C2H3) (1). Under the mild thermal experimental con ditions, the decarbonylation step may be sometimes a serious bottlenec k for the reaction to proceed, even to the extent of completely blocki ng the formation of the subsequent tetracarbonyl compound (CO)(4)Cr=C( OH)(C2H3) (5), which is supposed to add an alkyne molecule in the seco nd step. An alternative path is suggested, where the alkyne reacts dir ectly with the saturated metal-carbene complex 1 instead of competing with a CO molecule for a position in the coordination sphere of the me tal center in 5. Our calculations reveal that if alkyne addition takes place in 1 before CO loss, then the initial process becomes clearly e xothermic (-163.4 kJ mol-l). Moreover, this alternative path makes it easier for CO dissociation to occur as the second step because at this point the complex has more internal energy than 1 to expel a CO ligan d. From a comparison of both mechanistic routes, it is concluded that the Dotz reaction better proceeds initially through an associative ste p.